Q: How did they get caught?
A: The complaints against the Cowboys and the Redskins for engaging in perfectly legal behavior that violated no actual rules but only the collusive backroom dictates being issued by the league came not from the league office but from the other 30 teams, who were doing what they were told while the Cowboys and the Redskins were not. The other teams demanded action, since they were the good soldiers, and so the league decided it needed to take some.
Q: Why isn't the union challenging this?
A: While this behavior seems to fit the very dictionary definition of collusion, as multiple teams were engaged in discussions to limit the earning potential of their employees and prospective employees, do not expect it to be challenged in court. The decision today, in which the Redskins were docked $36 million in cap space and the Cowboys $10 million, is the result of a settlement between the NFL and the NFLPA. One reason the union has no problem with it is that the money lost to the Cowboys and Redskins is not taken out of the overall 2012 spending pool each of 28 other teams gets $1.6 million extra in cap room, so there's no net loss league-wide. Another reason the union won't push on it is because they agreed, as part of the settlement of last year's Brady vs. NFL federal lawsuit, to drop all pending legal action against the league. That included their claims that the league engaged in collusion in 2010.